Enormous groups of galaxies clue scientists in to the expansion of the universe.

When hot gas and millions of stars coalesce into galaxies, the gravity that glues these systems together may also recruit thousands of other galaxies to form one massive galaxy cluster. Stretching across millions of light-years and swarming with up to trillions of stars, galaxy clusters comprise the largest gravitationally bound objects in the known universe. Their sheer size and high composition of mysterious dark matter make them prized research subjects for astrophysicists. Using images of galaxy clusters from NASA’s Hubble Space Telescope, scientists have calculated that dark matter makes up approximately 23 percent of all matter and energy in space. These measurements impart clues into the ways dark matter may be driving the expansion of the universe after the Big Bang. Watch the video for a look inside a galaxy cluster.